MRI is an imaging technique in which a radio frequency (RF) pulse of Larmor frequency is emitted to cause magnetic excitation of a nucleus spin of an object placed in a static magnetic field, and nuclear magnetic resonance (NMR) signals are generated by this excitation to be used for reconstructing of images. In MRI, an RF coil is used to transmit an RF pulse to an imaging region to elicit nuclear magnetic resonance.
In this frequency range, the RF pulse causes an increase in body temperature of the object. Accordingly, from the viewpoint of safety, output power (RF pulse power) of the RF pulse to be transmitted to the object has an upper limit (limit value) specified by, for example, the International Electrotechnical Commission (IEC) standard or other standards. It is necessary to compute an absorption amount of RF pulse per unit time and per unit mass as a specific absorption rate (SAR), and to manage transmission so that SAR does not exceed the limit value. The SARs to be managed include a whole body SAR, a body part SAR, a head SAR, and a local SAR.
Specifically, output power (RF power) of an RF pulse signal supplied to the transmission coil is acquired, and the acquired RF power is used for SAR computing. A conventional whole body SAR (SARW) is computed by Expression (1) based on an RF power PW in a loaded condition, which are predicted based on an RF power obtained in prior imaging and imaging conditions, an RF power Pe in an unloaded condition, and an entire weight WW of the object. A conventional body part SAR (SARI) is computed by Expression (2) based on a body part PI of the RF power PW in the loaded condition, the RF power Pe in the unloaded condition, and a partial weight WI. A conventional head SAR (SARH) is computed by Expression (3) based on a head portion PH of the RF power PW in the loaded condition, the RF power Pe in the unloaded condition, and a head weight WH.
                                          SAR            W                    ⁡                      [                          W              ⁢                              /                            ⁢              kg                        ]                          =                                                            P                W                            ⁡                              [                W                ]                                      -                                          P                e                            ⁡                              [                W                ]                                                                        W              W                        ⁡                          [              kg              ]                                                          (        1        )                                                      SAR            I                    ⁡                      [                          W              ⁢                              /                            ⁢              kg                        ]                          =                                                            P                I                            ⁡                              [                W                ]                                      -                                          P                e                            ⁡                              [                W                ]                                                                        W              I                        ⁡                          [              kg              ]                                                          (        2        )                                                      SAR            H                    ⁡                      [                          W              ⁢                              /                            ⁢              kg                        ]                          =                                                            P                H                            ⁡                              [                W                ]                                      -                                          P                e                            ⁡                              [                W                ]                                                                        W              H                        ⁡                          [              kg              ]                                                          (        3        )            
When the whole body SAR, the body part SAR, and the head SAR are computed based on Expressions (1) to (3), the RF power Pe in the unloaded condition is used in each of the cases. As the RF power Pe in the unloaded condition, a fixed value obtained by measuring a pseudo human body model (phantom), which does not become a load as viewed from the transmission coil, is used.
A technology relating to an MRI apparatus which reduces artifacts and SARs is disclosed as a conventional technology relating to the present invention.
Although the RF power in the unloaded condition should be defined as an RF power absorbed by other than the object, a fixed value is used in the conventional technology. Accordingly, the fixed value may be inappropriate depending on a volume or a body fat percentage of the object (imaging region) due to an interaction between the transmission coil and the object, such as coupling therebetween. If the RF power in the unloaded condition is not appropriate, the SAR becomes too large and imaging is excessively limited, which results in a defect of deteriorated convenience, or the SAR becomes too small and imaging is permitted beyond limits, which results in a defect of deteriorated safety.